Conventionally, when performing an electronic shutter operation in a CMOS solid-state image sensor, object distortion and the like have occurred due to rolling scanning. This object distortion may prominently occur when the relationship between the scan time per screen (e.g., 1/10 sec if the scan rate is 10 frames/sec) and the shutter speed (e.g., 1/60 sec) for capturing the moving speed of the object is as follows:
shutter speed<scan time per screen
For this reason, technology has been developed for reducing the scan time per screen, that is to say, improving the scan rate so as to prevent the occurrence of object distortion even at higher shutter speeds. Also, in a product in which a rise in cost is permissible, technology in which a mechanical shutter is used along with a CMOS solid-state image sensor is employed, and object distortion is reduced regardless of the scan time per screen by mechanically obstructing light before performing rolling scanning.
Meanwhile, with solid-state image sensors used in digital cameras, digital video cameras, and the like in recent years, the decrease in size and increase in number of pixels as well as the increase in ISO sensitivity have been progressing, and thus it can be said that amplified small signals from the image sensor are being used. Since noise occurs when a small signal is amplified, it is critical to reduce the occurrence of noise at the same time as the optical sensitivity of the solid-state image sensor is increased.
For example, Japanese Patent Laid-Open No. 2006-246450 discloses a solid-state imaging device and a driving method for the same that enable the addition of functions for, for example, full-screen simultaneous holding and an increase in dynamic range while maintaining the saturation charge amount, by separating the photodiode unit and the holding unit, and furthermore providing the holding unit with leeway in terms of surface area. As shown in FIG. 5, a photodiode PD is connected to a holding unit Mem via a first transfer gate TX1, and a signal charge generated by the photodiode PD is transferred to the holding unit Mem from the start of the exposure period. In signal readout performed after the end of exposure, the signal is transferred from the holding unit Mem to a floating diffusion unit FD via a second transfer gate TX2, and thereafter rolling scanning is performed, which is unique to CMOS solid-state imaging devices. On the other hand, although the photodiode PD is exposed during the rolling scanning as well, the first transfer gate TX1 has been closed. Furthermore, the generated charge is constantly being output to an overflow drain OFD, and therefore there is no influence on the original signal charge in the holding unit Mem. In other words, although the signal readout scanning is rolling scanning, all of the pixels in the screen are controlled simultaneously by the first transfer gate TX1 from the start to the end of the actual holding of the signal charge, thus enabling full-screen simultaneous holding to be performed in principle.
With the above-described configuration, even if the surface area of the photodiode PD is relatively small, there is no particular influence on the photo-electric conversion characteristics if the efficiency with which light is condensed on the photodiode is improved. Instead, it has become possible to increase the surface area of the holding unit Mem, and maintain the saturation charge amount while transferring the signal charge from the start of the exposure period.
Meanwhile, a configuration of a solid-state image sensor is known in which the dark current component is reduced by controlling the potential below the gate electrode of a transfer MOS transistor, thus achieving a hole storage state (see Japanese Patent Laid-Open No. 2002-247456, for example).
However, with the technology disclosed in the above-described Japanese Patent Laid-Open No. 2006-246450, a buried channel of the transfer MOS transistor between the photodiode PD and the holding unit Mem serves as a countermeasure for an increase in dark current that occurs due to the continuous rise in the channel potential of the transfer MOS transistor during exposure. Therefore, this requires advanced manufacturing technology.